1. Field of the Invention
The present invention relates to high speed apparatus for measuring the thickness of a container wall employing a plurality of sensor elements which combine to inspect the container and to an associated method.
2. Description of the Prior Art
The importance of inspecting containers, such as glass, or plastic bottles, or jars, in order to make sure that undesired thickness does not result in problems during filling, handling, consumer use, and refilling has long been known. The desired thickness is a function of the material out of which the container is made, the contemplated contents of the container and other factors involved in the handling, filling, use, and refilling of the container.
The variations in thickness can result from such factors as temperature variations in the mold, non-uniform final blow pressure and other process related functions, and also mold seams, i.e., the regions where the two molds used to make the bottle in the forming machine have been joined.
In recent years, new manufacturing techniques have been developed that can produce thinner containers while maintaining the inherent strength of glass containers. As to such containers, it is even more critical that the thickness does not fall below a predetermined desired value.
As to inspections of this type, it is necessary to inspect every container rather than relying on a sampling technique wherein only a selected percentage of the total number of containers are inspected.
It has been known to use changes in capacitance which are subsequently converted to corresponding voltages relating to wall thickness by an oscillator means. See generally U.S. Pat. Nos. 4,820,972; 4,862,062; 4,870,342; and 4,888,824.
In U.S. Pat. No. 4,862,062, there is disclosed a system wherein a number of changes in capacitance are measured about the periphery of the wall of a container as a result of the container being urged into intimate contact with and then rolling along single elongated sensors positioned at three levels. The capacitance is then converted into a voltage which is used to determine wall thickness. The sensors or probes are typically flexible in order to enable the bottle to be urged into intimate contact with the sensor as by a feed screw shown in this patent. Generally similar systems are shown in U.S. Pat. Nos. 4,870,342; 4,820,972; and 4,888,824. One of the difficulties with this type of system is the fact that such inspection devices are relatively slow as the equipment can handle only a single container at a time. The actual throughput speed in containers per minute will vary depending upon the circumferential size of the container. As the speeds of these types of inspection systems, which may be on the order of 100 to 300 bottles per minute, is significantly less than the speed of current bottle forming machines, it is generally necessary to associate two or three such devices with each forming machine in order to effect inspection of all of the containers. This results in additional costs of equipment and associated interconnecting apparatus, such as conveyors, and the need to maintain additional equipment. A modern forming machine may well produce containers, depending on size, at a rate of 500 a minute or more.
It will be appreciated, therefore, that there remains a real and substantial need for a container wall thickness measuring device which is capable of accurately measuring container wall thickness at a high speed.